The present application relates to a fuel cell and an electronic device equipped therewith, the fuel cell being characterized in that at least either of its cathode or anode has an enzyme as a catalyst immobilized thereon.
A fuel cell typically includes a cathode (oxidizer electrode) and an anode (fuel electrode), which face each other with an electrolyte (proton conductor) interposed between them. It generates electric energy by the following mechanism. The anode is supplied with fuel (hydrogen), which is oxidized and decomposed into electrons and protons (H+), with the former moving to the anode and the latter moving to the cathode through the electrolyte. The cathode is supplied with oxygen from the outside and electrons from the anode through an external circuit. Reaction takes place on the cathode between oxygen, protons, and electrons to give H2O.
Being based on the foregoing principle, a fuel cell is an efficient electric power generator that converts fuel's chemical energy directly into electric energy. In other words, it generates electric energy efficiently from chemical energy possessed by fossil fuel such as natural gas, petroleum, and coal at any time and any place. For this reason, active research and development works have been made to exploit fuel cells as large-scale power generators. An example of successful attempts is the fuel cell mounted on the space shuttle, which not only generated electric power but also supplied crew with water without environmental pollution.
Among recent notable achievements is the development of fuel cells of solid polymer type which operates at comparatively low temperatures ranging from room temperature to about 90° C. Such fuel cells are expected to find use not only as large-scale power generators but also as small-scale portable power generators for automobiles and personal computers.
Thus, fuel cells are attracting attention for their possible use as efficient power generators that operate on any scale. Unfortunately, they still involve many problems. That is, they consume limited global resources so long as they rely on hydrogen gas converted from natural gas, petroleum, and coal through a reformer. They need heating at high temperatures, and they also need catalysts of expensive precious metal such as platinum (Pt). In addition, their fuel, such as hydrogen gas and methanol, requires handling precautions.
With a view to addressing the above-mentioned problems, there has been proposed a fuel cell based on the principle of the metabolism in living organisms which is a highly efficient energy conversion mechanism. The term “metabolism” embraces respiration and photosynthesis that take place in the cells of microorganisms. Metabolism in living organisms performs very efficient power generation and proceeds under mild conditions at room temperature.
Respiration is made up of intake of nutrients (such as saccharides, fats, and proteins) into microorganisms or cells, enzymatic reactions to form carbon dioxide (CO2) through the glycolytic pathway and tricarboxylic acid (TCA) cycle, reduction to convert nicotinamide adenine dinucleotide (NAD+) into nicotinamide adenine dinucleotide in reduced form (NADH), thereby generating oxidation-reduction energy or electric energy, direct conversion of the electric energy of NADH into the electric energy of proton gradient in the electron transport system, and reduction of oxygen into water. The thus generated electric energy forms ATP from adenosine diphosphate (ADP) with the help of adenosine triphosphate (ATP) synthetase, and the resulting ATP is used for reactions that help the growth of microorganisms and cells. This energy conversion takes place in cytosol and mitochondria.
Photosynthesis is a mechanism to generate electric energy from light energy by reduction of nicotinamide adenine dinucleotide phosphate (NADP+) into nicotinamide adenine dinucleotide phosphate (NADPH) in reduced form through the electron transport system, with evolution of oxygen by oxidation of water. The resulting electric energy is used for carbon assimilation from CO2 and synthesis of carbohydrates.
Efforts to apply the metabolism in living organisms to fuel cells has been realized in the microorganism battery (reported in Japanese Unexamined Patent Application Publication (JP-A) No. 2000-133297) which causes microorganisms to generate electric energy and sends the thus generated electric energy (electrons) to electrodes outside microorganisms through an electron mediator, so that the battery supplies electric current.
However, conversion of chemical energy into electric energy by microorganisms and cells is not sufficiently efficient because it involves many unnecessary reactions other than desired ones. Thus the above-mentioned method is poor in energy conversion efficiency.
On the other hand, there has been proposed a bio-fuel cell which permits only desired reactions to take place with the help of an enzyme, as disclosed in JP-A-2003-282124, JP-A-2004-71559, and JP-A-2005-13210. It is so designed as to decompose fuel into protons and electrons by means of an enzyme. Its fuel includes alcohols (such as methanol and ethanol) and monosaccharides such as glucose.